Varactors are voltage tunable capacitors in which the capacitance is dependent on a voltage applied thereto. Although not limited in this respect, this property has applications in electrically tuning radio frequency (RF) circuits, such as filters, phase shifters, and so on. The most commonly used varactor is a semiconductor diode varactor, which has the advantages of high tunability and low tuning voltage, but suffers low Q, low power handling capability, and limited capacitance range. A new type of varactor is a ferroelectric varactor in which the capacitance is tuned by varying the dielectric constant of a ferroelectric material by changing the bias voltage. Ferroelectric varactors have high Q, high power handling capacity, and high capacitance range.
One ferroelectric varactor is disclosed in U.S. Pat. No. 5,640,042 entitled “Thin Film Ferroelectric Varactor” by Thomas E. Koscica et al. That patent discloses a planar ferroelectric varactor, which includes a carrier substrate layer, a high temperature superconducting metallic layer deposited on the substrate, a lattice matching, a thin film ferroelectric layer deposited on the metallic layer, and a plurality of metallic conductors disposed on the ferroelectric layer and in contact with radio frequency (RF) transmission lines in tuning devices. Another tunable capacitor using a ferroelectric element in combination with a superconducting element is disclosed in U.S. Pat. No. 5,721,194. Tunable varactors that utilize a ferroelectric layer, and various devices that include such varactors are also disclosed in U.S. Pat. No. 6,531,936, entitled “Voltage Tunable Varactors And Tunable Devices Including Such Varactors,” filed Oct. 15, 1999, and assigned to the same assignee as the present invention.
Tunable filters are vital to myriad devices. Further, performance improvements are constantly needed and it would advantageous to meet performance requirements such as but not limited to: Less than 3 mm×3 mm×1 mm in size, $0.20 per unit volume production cost, Multi-pole band-pass filter response, Less than 10% 3 dB bandwidth, More than 20% tuning range, Less than 4 dB insertion loss, Higher than 40 dBm Third Order Intercept (IP3).
Previously, attempts to improve tunable filters incorporated fixed capacitors and inductors, bulk acoustic wave resonators, discrete air coils as inductors, distributed transmission line type inductors or resonators and dielectric block resonators. However, these previous attempts at tunable filter performance and size improvements have the following limitations:
a. Fixed capacitors and inductors: No tunability
b. Bulk acoustic wave resonators: Very small tuning range
c. Discrete air coils as inductors: Large size, low Q-factor, high cost
d. Distributed transmission line type inductors or resonators: Large size
e. Dielectric block resonators: Very small tuning range
Thus, a strong need exists for a compact, improved performance tunable filter and method of operation and manufacture therefore